The present invention relates to an optical radar apparatus in which light is emitted from a light-emitting device and is transmitted forward, and more particularly to an optical radar apparatus in which horizontal scanning is effected with emitted light, and reflected light from an object to be measured is received to detect the distance to and the direction of the object to be measured.
FIG. 7 is a schematic diagram illustrating a light-emitting device used in a conventional optical radar apparatus for a vehicle. A light-emitting device 110 shown in FIG. 7 has a laser diode (LD) 32 mounted for emitting a laser beam, and a semiconductor light-emitting element (LD chip) 32 and a base (stay) 33 for holding the semiconductor light-emitting element 32 are accommodated in a laser holder 31. In addition, a cover glass 34 for sealing the interior of the laser holder 31 is provided at the front surface, as viewed in the light-emitting direction, of the semiconductor light-emitting element 32.
Further, a lens 3 is provided in front, as viewed in the light-emitting direction, of the semiconductor light-emitting element 32, and this lens 3 performs the function of adjusting the laser light to a predetermined broadening angle. Component of the light transmission system such as the light-emitting device 110 and the lens 3 are mounted and held on a main board 14 by means of a lens holder 2.
FIG. 8 is a perspective view illustrating a fixed-mirror assembly 40 which performs the function of reflecting a light beam from the light-emitting device 110 and changing its direction, and this fixed-mirror assembly 40 is comprised of a mirror 21 and a supporting member 22 for supporting the mirror 21.
FIG. 9 is a diagram of the positional relationship illustrating the light transmitting direction of the light beam from the light-emitting device 110. In the drawing, the light beam emitted from the light-emitting device 110 is transmitted to a light-transmitting mirror 5 via the fixed mirror assembly 40. Then, the light beam reflected by the fixed mirror assembly 40 undergoes a change in its direction by the light-transmitting mirror 5, and is radiated in the forward direction.
The light-emitting device and the light transmission system used in the conventional optical radar apparatus are arranged as described above, and have problems which are described below.
First, in the fixed-mirror assembly 40 shown in FIG. 8, the supporting member 22 for supporting the mirror 21 has a shape in which a plate is bent into an L-shape. Namely, the overall surface for attaching the mirror 21 has a planar shape, and deflection is liable to occur in that portion. If the mirror 21 is attached in that state, the mirror 21 also becomes deflected along the deflection of the mirror 21. As a result, the light beam formed in to a predetermined beam shape by means of the lens 3 is reflected by the mirror 21 having the deflected reflecting surface, and is radiated in a dispersed manner as in the directions shown by the broken lines in FIG. 9.
In addition, with the conventional optical radar apparatus, the laser diode is widely used as the light-emitting means as shown in FIG. 7, but the material of the stay 33 and the holder 31 is generally a metal, and the inner peripheral surface and the bottom surface of the holder 31 are provided with gold plating or the like. For this reason, as shown in FIG. 7, part of the light beam emitted from the semiconductor light-emitting element (LD chip) 32 is reflected by, for instance, the inner surface of the cover glass 34, is reflected again by the bottom surface of the holder 31, is transmitted through the cover glass 34 as shown by the broken lines, and is emitted. This light beam is apparently similar to the light beam emitted from a position remote from the focal position. If such a light beam enters the lens 3, the light beam is deflected in the directions shown by the broken lines in FIG. 7. Then, the light beam becomes divergent light in which, as the light beam moves away from the lens 3, the light beam once concentrates the main light beam and then proceeds in a dispersive direction.
Here, a laser light-emitting device disclosed in Unexamined Japanese Patent Publication 8-5744/(1996) has a reflection preventing means which is provided only on the surface of the stay for holding the LD chip, but this reflection preventing means prevents reflection of incident light from other than the LD chip. According to this method, since the inner peripheral surface and the bottom surface of the holder are not provided with any coating, the light formed by part of the light beam which was emitted from the LD chip and reached the inner peripheral surface and the bottom surface of the holder becomes similar to the one described above.
In a short distance, the divergent light beam such as the one shown by the broken lines in FIGS. 7 and 9 is also radiated to an object located outside the substantially parallel light beam emitted from the lens shown by the solid lines in FIGS. 7 and 9. In an optical radar apparatus for a vehicle, in particular, in a case where this light beam radiated to the outside is large in intensity and is reflected by an object having a large reflectivity, strong reflected light is made incident upon a light-receiving element 8 via a light-receiving mirror 6 and a light-receiving lens 7 which will be described later.
Accordingly, this strong reflected light caused by the light beam reflected to the outside exceeds the threshold of the light-receiving sensitivity of the light-receiving element 8 (which will be described later), so that an erroneous detection is made that the object is present in the forward direction shown by the dotted-dash line in FIG. 1 (which will be described later). Thus, there is a problem in that, in a short distance, such a light beam which is radiated in a slightly dispersed manner in the vicinity of the light beam is also detected when it is reflected by a large reflecting plate or the like installed at the shoulder of a road or the like having a large reflectivity and a large area, thereby making it impossible to detect an actual object to be detected.